This invention relates to novel aluminosilicate desiccants and to their manufacture by controlled acid treatment of hydroxysodalite. The invention also relates to the preferred use of the desiccants in enthalpy air to air rotary energy exchangers.
Ambient air contains two separate substantial energy components: sensible (temperature) and latent humidity) heat. Enthalpy or total energy recovery involves combining heat exchange with latent energy or humidity exchange. Conventional heat exchange is well-established and widely practiced. This procedure effectively exchanges sensible heat and may serve as a barrier which transfers sensible heat between two streams. This may be accomplished by a variety of techniques including a turning wheel. The concept of enthalpy exchange or total energy recovery involves simultaneous sensible and latent heat transfer. This requires applying a desiccant to a heat exchange system to transfer that latent component by adsorbing and desorbing humidity. This combined energy transfer is particularly well-suited for a turning wheel device. If a completely effective enthalpy wheel were available, outdoor air could be drawn into a room while xe2x80x9cstalexe2x80x9d air was exhausted without changing the interior latent or sensible energy levels. The practical result would be fresh air in the summer without additional cooling and fresh air in the winter without additional heating.
In order for a desiccant to be applied to an enthalpy recovery system, it must be extremely responsive to temperature and humidity changes near ambient conditions. Additionally, it should not adsorb molecules larger than water, in that their exchange could result in a build-up of contaminants in the conditioning space. The current desiccant of choice for enthalpy recovery applications is Zeolite 3A. The teachings of U.S. Pat. No. 4,769,053 (Fisher) are incorporated herein by cross-reference. A non-classical zeolite, titanium silicate ETS-10 supplied by Engelhard Corporation, has recently been used for this application.
In principle, Zeolite 3A may act as a barrier for moisture between indoor and outdoor environments by acting as a reversible desiccant. Unfortunately, as would be expected from classical aluminosilicate zeolites, Zeolite 3A binds water very strongly and would be expected to xe2x80x9cswingxe2x80x9d negligible amounts of water under the very mild temperature and humidity perturbations seen by an enthalpy wheel. Thus, while Zeolite 3A may exclude most species larger than water, its basic desiccant characteristics leave much to be desired for these types of applications. While expressing more desirable desiccant properties than Zeolite 3A, if absolute exclusion of other molecules is desired, the approximately 8 Angstrom pore size of ETS-10 is too large. In principle, a wide spectrum of molecules may fit through its relatively large pore and be retained by the adsorbent.
We have discovered that hydroxysodalite, a dense aluminosilicate having pores smaller than the 2.8 Angstrom diameter of water, can be transformed into a novel less dense macroporous structure by a controlled acid treatment.
The novel macroporous structure is a material that still contains sodalite cages but has also formed macropores that have a broad range of pore sizes including many large pores.
In fact, nitrogen sorption tests show that the novel acid treated sodalite of this invention contains pores ranging from 20 to 500 Angstroms in diameter. The sample shown in the pore size distribution data in FIG. 2 has a cumulative pore volume of 0.12 cc/g between said 20 to 500 Angstroms pore diameter. Half of this pore volume is in pores greater than 150 Angstroms diameter. There is an additional 0.03 cc/g more pore volume between 500 to 2500 Angstroms.
Other acid treated sodalite samples of this invention have a cumulative pore volume between 20 to 500 Angstrom diameters of at least 0.04 cc/g, preferably at least 0.08 cc/g and as high as 0.15 cc/g. This contrasts to sodalite with 0.006 cc/g and zeolite A with 0.002 cc/g over the same range. High surface area silica (448 m2/g ASTM standard) has a higher pore volume over the 20 to 500 Angstroms diameter range, but a very different pore size distribution. More than 90% of its pore volume is in pores less than 150 Angstroms in diameter, with no measured additional pore volume between 500 to 2500 Angstrom diameter pores.
These large pores are responsible for causing fast water xe2x80x9cswingxe2x80x9d capacity at low temperatures making said novel material eminently suitable for use in air to air rotary energy exchanges.
This is surprising since, as well known, sodalite sorbs water only in its zeolite cages and only very slowly.